Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT 2017gfo


The gravitational-wave event GW 170817 was generated by the coalescence of two neutron stars and produced an electromagnetic transient, labelled AT 2017gfo, that was the target of a massive observational campaign. Polarimetry is a powerful diagnostic tool for probing the geometry and emission processes of unresolved sources, and the observed linear polarization for this event was consistent with being mostly induced by intervening dust, suggesting that the intrinsic emission was weakly polarized (P < 0.4–0.5%). Here we present a detailed analysis of the linear polarization expected from a merging neutron-star binary system by means of 3D Monte Carlo radiative transfer simulations assuming a range of possible configurations, wavelengths, epochs and viewing angles. We find that polarization originates from the non-homogeneous opacity distribution within the ejecta and can reach levels of 1% at early times (one to two days after the merger) and in the optical R band. Smaller polarization signals are expected at later epochs and different wavelengths. From the viewing-angle dependence of the polarimetric signal, we constrain the observer orientation of AT 2017gfo to within about 65° from the polar direction. The detection of non-zero polarization in future events will unambiguously reveal the presence of a lanthanide-free ejecta component and unveil its spatial and angular distribution.

This is a preview of subscription content

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Sketch of the fiducial kilonova model used in this work.
Fig. 2: Predicted linear polarization at 1.5 days and at 7,000 Å as a function of the viewing angle of the system, θobs.
Fig. 3: Linear polarization Q for an equatorial viewing angle (cosθobs = 0) at different wavelengths and epochs.
Fig. 4: Impact of the half-opening angle of the lanthanide-rich ejecta, Φ, on the polarization signal predicted at 7,000 Å and 1.5 days after the merger.
Fig. 5: Opacities for the lanthanide-free component at high latitudes (blue) and lanthanide-rich component at low latitudes (red) for various times after the merger.
Fig. 6: Bound–bound opacities used in the simulations as a function of time since the merger.


  1. 1.

    Abbott, B. P. et al. GW170817: observation of gravitational waves from a binary neutron star inspiral. Phys. Rev. Lett. 119, 161101 (2017).

    ADS  Article  Google Scholar 

  2. 2.

    Abbott, B. P. et al. Multi-messenger observations of a binary neutron star merger. Astrophys. J. 848, L12 (2017).

    ADS  Article  Google Scholar 

  3. 3.

    Mooley, K. P. et al. Superluminal motion of a relativistic jet in the neutron star merger GW170817. Preprint at (2018).

  4. 4.

    Ghirlanda, G. et al. Re-solving the jet/cocoon riddle of the first gravitational wave with an electromagnetic counterpart. Preprint at (2018).

  5. 5.

    Covino, S. et al. The unpolarized macronova associated with the gravitational wave event GW 170817. Nature Astron. 1, 791–794 (2017).

    ADS  Article  Google Scholar 

  6. 6.

    Evans, P. A. et al. Swift and NuSTAR observations of GW170817: detection of a blue kilonova. Science 358, 1565–1570 (2017).

    ADS  Article  Google Scholar 

  7. 7.

    Pian, E. et al. Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger. Nature 551, 67–70 (2017).

    ADS  Google Scholar 

  8. 8.

    Smartt, S. J. et al. A kilonova as the electromagnetic counterpart to a gravitational-wave source. Nature 551, 75–79 (2017).

    ADS  Google Scholar 

  9. 9.

    Tanvir, N. R. et al. The emergence of a lanthanide-rich kilonova following the merger of two neutron stars. Astrophys. J. 848, L27 (2017).

    ADS  Article  Google Scholar 

  10. 10.

    Li, L.-X. & Paczyński, B. Transient events from neutron star mergers. Astrophys. J. 507, L59–L62 (1998).

    ADS  Article  Google Scholar 

  11. 11.

    Metzger, B. D. et al. Electromagnetic counterparts of compact object mergers powered by the radioactive decay of r-process nuclei. Mon. Not. R. Astron. Soc. 406, 2650–2662 (2010).

    ADS  Article  Google Scholar 

  12. 12.

    Roberts, L. F., Kasen, D., Lee, W. H. & Ramirez-Ruiz, E. Electromagnetic transients powered by nuclear decay in the tidal tails of coalescing compact binaries. Astrophys. J. 736, L21 (2011).

    ADS  Article  Google Scholar 

  13. 13.

    Kasen, D., Badnell, N. R. & Barnes, J. Opacities and spectra of the r-process ejecta from neutron star mergers. Astrophys. J. 774, 25 (2013).

    ADS  Article  Google Scholar 

  14. 14.

    Barnes, J. & Kasen, D. Effect of a high opacity on the light curves of radioactively powered transients from compact object mergers. Astrophys. J. 775, 18 (2013).

    ADS  Article  Google Scholar 

  15. 15.

    Tanaka, M. & Hotokezaka, K. Radiative transfer simulations of neutron star merger ejecta. Astrophys. J. 775, 113 (2013).

    ADS  Article  Google Scholar 

  16. 16.

    Baiotti, L. & Rezzolla, L. Binary neutron star mergers: a review of Einstein’s richest laboratory. Rep. Prog. Phys. 80, 096901 (2017).

    ADS  MathSciNet  Article  Google Scholar 

  17. 17.

    Metzger, B. D. Kilonovae. Liv. Rev. Rel. 20, 3 (2017).

    Article  Google Scholar 

  18. 18.

    Tanaka, M. et al. Kilonova from post-merger ejecta as an optical and near-Infrared counterpart of GW170817. PASJ 69, 102 (2017).

    ADS  Google Scholar 

  19. 19.

    Tanaka, M. et al. Properties of kilonovae from dynamical and post-merger ejecta of neutron star mergers. Astrophys. J. 852, 109 (2018).

    ADS  Article  Google Scholar 

  20. 20.

    Kyutoku, K., Ioka, K. & Shibata, M. Anisotropic mass ejection from black hole-neutron star binaries: diversity of electromagnetic counterparts. Phys. Rev. D 88, 041503 (2013).

    ADS  Article  Google Scholar 

  21. 21.

    Kyutoku, K., Ioka, K., Okawa, H., Shibata, M. & Taniguchi, K. Dynamical mass ejection from black hole-neutron star binaries. Phys. Rev. D 92, 044028 (2015).

    ADS  Article  Google Scholar 

  22. 22.

    Hotokezaka, K. et al. Mass ejection from the merger of binary neutron stars. Phys. Rev. D 87, 024001 (2013).

    ADS  Article  Google Scholar 

  23. 23.

    Bauswein, A., Goriely, S. & Janka, H.-T. Systematics of dynamical mass ejection, nucleosynthesis, and radioactively powered electromagnetic signals from neutron-star mergers. Astrophys. J. 773, 78 (2013).

    ADS  Article  Google Scholar 

  24. 24.

    Fernández, R. & Metzger, B. D. Delayed outflows from black hole accretion tori following neutron star binary coalescence. Mon. Not. R. Astron. Soc. 435, 502–517 (2013).

    ADS  Article  Google Scholar 

  25. 25.

    Siegel, D. M. & Metzger, B. D. Three-dimensional general-relativistic magnetohydrodynamic simulations of remnant accretion disks from neutron star mergers: outflows and r-process nucleosynthesis. Phys. Rev. Lett. 119, 231102 (2017).

    ADS  Article  Google Scholar 

  26. 26.

    Shibata, M., Kiuchi, K. & Sekiguchi, Y.-i General relativistic viscous hydrodynamics of differentially rotating neutron stars. Phys. Rev. D 95, 083005 (2017).

    ADS  Article  Google Scholar 

  27. 27.

    Fujibayashi, S., Sekiguchi, Y., Kiuchi, K. & Shibata, M. Properties of neutrino-driven ejecta from the remnant of binary neutron star merger: purely radiation hydrodynamics case. Preprint at (2017).

  28. 28.

    Metzger, B. D. & Fernández, R. Red or blue? A potential kilonova imprint of the delay until black hole formation following a neutron star merger. Mon. Not. R. Astron. Soc. 441, 3444–3453 (2014).

    ADS  Article  Google Scholar 

  29. 29.

    Lippuner, J. et al. Signatures of hypermassive neutron star lifetimes on r-process nucleosynthesis in the disc ejecta from neutron star mergers. Mon. Not. R. Astron. Soc. 472, 904–918 (2017).

    ADS  Article  Google Scholar 

  30. 30.

    Perego, A. et al. Neutrino-driven winds from neutron star merger remnants. Mon. Not. R. Astron. Soc. 443, 3134–3156 (2014).

    ADS  Article  Google Scholar 

  31. 31.

    Chornock, R. et al. The electromagnetic counterpart of the binary neutron star merger LIGO/Virgo GW170817. IV. Detection of near-infrared signatures of r-process nucleosynthesis with gemini-south. Astrophys. J. 848, L19 (2017).

    ADS  Article  Google Scholar 

  32. 32.

    Cowperthwaite, P. S. et al. The electromagnetic counterpart of the binary neutron star merger LIGO/Virgo GW170817. II. UV, optical, and near-infrared light curves and comparison to kilonova models. Astrophys. J. 848, L17 (2017).

    ADS  Article  Google Scholar 

  33. 33.

    Nicholl, M. et al. The electromagnetic counterpart of the binary neutron star merger LIGO/Virgo GW170817. III. optical and UV spectra of a blue kilonova from fast polar ejecta. Astrophys. J. 848, L18 (2017).

    ADS  Article  Google Scholar 

  34. 34.

    Bulla, M., Sim, S. A. & Kromer, M. Polarization spectral synthesis for type Ia supernova explosion models. Mon. Not. R. Astron. Soc. 450, 967–981 (2015).

    ADS  Article  Google Scholar 

  35. 35.

    Bulla, M. Polarisation Spectral Synthesis For Type Ia Supernova Explosion Models PhD thesis, Queen’s University Belfast (2017).

  36. 36.

    Serkowski, K., Mathewson, D. S. & Ford, V. L. Wavelength dependence of interstellar polarization and ratio of total to selective extinction. Astrophys. J. 196, 261–290 (1975).

    ADS  Article  Google Scholar 

  37. 37.

    Wang, L. & Wheeler, J. C. Spectropolarimetry of Supernovae. Ann. Rev. Astron. Astrophys. 46, 433–474 (2008).

    ADS  Article  Google Scholar 

  38. 38.

    Maund, J. R. et al. Spectropolarimetry of the type IIb Supernova 2001ig. Astrophys. J. 671, 1944–1958 (2007).

    ADS  Article  Google Scholar 

  39. 39.

    Patat, F. et al. VLT Spectropolarimetry of the type Ia SN 2005ke. A step towards understanding subluminous events. Astron. Astrophys. 545, A7 (2012).

    Article  Google Scholar 

  40. 40.

    Troja, E. et al. The X-ray counterpart to the gravitational-wave event GW 170817. Nature 551, 71–74 (2017).

    ADS  Article  Google Scholar 

  41. 41.

    Finstad, D., De, S., Brown, D. A., Berger, E. & Biwer, C. M. Measuring the viewing angle of GW170817 with electromagnetic and gravitational waves. Preprint at (2018).

  42. 42.

    Mandel, I. The orbit of GW170817 was inclined by less than 28 deg to the line of sight. Astrophys. J. 853, L12 (2018).

    ADS  Article  Google Scholar 

  43. 43.

    Metzger, B. D., Thompson, T. A. & Quataert, E. A magnetar origin for the kilonova ejecta in GW170817. Astrophys. J. 856, 101 (2018).

    ADS  Article  Google Scholar 

  44. 44.

    D’Avanzo, P. et al. The evolution of the X-ray afterglow emission of GW 170817 / GRB 170817A in XMM-Newton observations. Preprint at (2018).

  45. 45.

    Kasen, D., Fernández, R. & Metzger, B. D. Kilonova light curves from the disc wind outflows of compact object mergers. Mon. Not. R. Astron. Soc. 450, 1777–1786 (2015).

    ADS  Article  Google Scholar 

  46. 46.

    Kasen, D., Metzger, B., Barnes, J., Quataert, E. & Ramirez-Ruiz, E. Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event. Nature 551, 80–84 (2017).

    ADS  Google Scholar 

  47. 47.

    Shibata, M. et al. Modeling GW170817 based on numerical relativity and its implications. Phys. Rev. D 96, 123012 (2017).

    ADS  Article  Google Scholar 

  48. 48.

    Inserra, C., Bulla, M., Sim, S. A. & Smartt, S. J. Spectropolarimetry of superluminous supernovae: insight into their geometry. Astrophys. J. 831, 79 (2016).

    ADS  Article  Google Scholar 

  49. 49.

    Jeffery, D. J. The Sobolev-P method—a generalization of the Sobolev method for the treatment of the polarization state of radiation and the polarizing effect of resonance line scattering. Astrophys. J. Suppl. Ser. 71, 951–981 (1989).

    ADS  Article  Google Scholar 

  50. 50.

    Mazzali, P. A. & Lucy, L. B. The application of Monte Carlo methods to the synthesis of early-time supernovae spectra. Astron. Astrophys. 279, 447–456 (1993).

    ADS  Google Scholar 

  51. 51.

    Chandrasekhar, S. Radiative Transfer (Dover Publications, New York, 1960).

    MATH  Google Scholar 

  52. 52.

    Code, A. D. & Whitney, B. A. Polarization from scattering in blobs. Astrophys. J. 441, 400–407 (1995).

    ADS  Article  Google Scholar 

  53. 53.

    Wang, L., Wheeler, J. C. & Höflich, P. Polarimetry of the type IA supernova SN 1996X. Astrophys. J. 476, L27–L30 (1997).

    ADS  Article  Google Scholar 

  54. 54.

    Hoflich, P. Asphericity effects in scattering dominated photospheres. Astron. Astrophys. 246, 481 (1991).

    ADS  Google Scholar 

  55. 55.

    Martin, D. et al. Neutrino-driven winds in the aftermath of a neutron star merger: nucleosynthesis and electromagnetic transients. Astrophys. J. 813, 2 (2015).

    ADS  Article  Google Scholar 

  56. 56.

    Bovard, L. et al. r -process nucleosynthesis from matter ejected in binary neutron star mergers. Phys. Rev. D 96, 124005 (2017).

    ADS  Article  Google Scholar 

  57. 57.

    Hillier, D. J. The calculation of continuum polarization due to the Rayleigh scattering phase matrix in multi-scattering axisymmetric envelopes. Astron. Astrophys. 289, 492–504 (1994).

    ADS  Google Scholar 

  58. 58.

    Wood, K., Bjorkman, J. E., Whitney, B. & Code, A. The effect of multiple scattering on the polarization from axisymmetric circumstellar envelopes. II. Thomson scattering in the presence of absorptive opacity sources. Astrophys. J. 461, 847 (1996).

    ADS  Article  Google Scholar 

  59. 59.

    Kasen, D. et al. Analysis of the flux and polarization spectra of the type Ia supernova SN 2001el: exploring the geometry of the high-velocity ejecta. Astrophys. J. 593, 788–808 (2003).

    ADS  Article  Google Scholar 

  60. 60.

    Dessart, L. & Hillier, D. J. Synthetic line and continuum linear-polarization signatures of axisymmetric type II supernova ejecta. Mon. Not. R. Astron. Soc. 415, 3497–3519 (2011).

    ADS  Article  Google Scholar 

  61. 61.

    Kasen, D., Nugent, P., Thomas, R. C. & Wang, L. Could there be a hole in type Ia supernovae? Astrophys. J. 610, 876–887 (2004).

    ADS  Article  Google Scholar 

  62. 62.

    Bulla, M. et al. Predicting polarization signatures for double-detonation and delayed-detonation models of type Ia supernovae. Mon. Not. R. Astron. Soc. 462, 1039–1056 (2016).

    ADS  Article  Google Scholar 

  63. 63.

    Plaszczynski, S., Montier, L., Levrier, F. & Tristram, M. A novel estimator of the polarization amplitude from normally distributed Stokes parameters. Mon. Not. R. Astron. Soc. 439, 4048–4056 (2014).

    ADS  Article  Google Scholar 

Download references


M.B. acknowledges support from the Swedish Research Council (Vetenskapsrå det) and the Swedish National Space Board. S.C. acknowledges support from ASI grant I/004/11/3 and partial financial support by the GRAWITA collaboration. K.K. is supported by the Japanese Society for the Promotion of Science (JSPS) Kakenhi Grant-in-Aid for Scientific Research (grant numbers JP16H06342, JP17H01131 and JP18H04595). J.R.M. is supported through a Royal Society University Research Fellowship. K.T. is supported by JSPS Kakenhi grant numbers 15H05437 and 18H01245, and also by a JST grant 'Building of Consortia for the Development of Human Resources in Science and Technology'. J.B. is supported by a University of Sheffield PhD studentship.

Author information




All authors contributed to the work presented in this paper. M.B. carried out the model simulations and analysis and led the writing of the manuscript. S.C. provided the polarimetric data of AT 2017gfo and helped with the writing. K.K. and M.T. provided theoretical insights on hydrodynamical models, carried out radiative transfer calculations to estimate opacities and helped with the writing of the manuscript.

Corresponding authors

Correspondence to M. Bulla or S. Covino.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Bulla, M., Covino, S., Kyutoku, K. et al. The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT 2017gfo. Nat Astron 3, 99–106 (2019).

Download citation

Further reading


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing